Smart composites containing modified cellulosic nanomaterials

ABSTRACT

In accordance with some embodiments of the present invention, a composite material is prepared by blending a bio-derived filler into a polymer, wherein the filler includes a diene-modified cellulosic nanomaterial (e.g., cellulose nanocrystals (CNCs) and/or cellulose nanofibrils (CNFs) functionalized to contain a diene) and a dienophile-modified cellulosic nanomaterial (e.g., CNCs and/or CNFs functionalized to contain a dienophile). The modulus of the composite material is reversibly controllable by adjusting a degree of crosslinking between the diene-modified cellulosic nanomaterial and the dienophile-modified cellulosic nanomaterial. This degree of crosslinking is thermally reversible. On one hand, the degree of crosslinking may be increased via a Diels-Alder (DA) cycloaddition reaction at a first temperature, thereby increasing the modulus of the composite material. On the other hand, the degree of crosslinking may be decreased via a retro-DA reaction at a second temperature higher than the first temperature, thereby decreasing the modulus of the composite material.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application is a divisional application of pending U.S.patent application Ser. No. 14/203,975, filed Mar. 11, 2014, entitled“SMART COMPOSITES CONTAINING MODIFIED CELLULOSIC NANOMATERIALS”, whichis hereby incorporated herein by reference in its entirety.

BACKGROUND

The present invention relates in general to the field of materialsscience. More particularly, the present invention relates to compositematerials having a reversibly controllable modulus and containing abio-derived filler blended with a polymer, wherein the filler includes adiene-modified cellulosic nanomaterial and a dienophile-modifiedcellulosic nanomaterial.

SUMMARY

In accordance with some embodiments of the present invention, acomposite material is prepared by blending a bio-derived filler into apolymer, wherein the filler includes a diene-modified cellulosicnanomaterial (e.g., cellulose nanocrystals (CNCs) and/or cellulosenanofibrils (CNFs) functionalized to contain a diene) and adienophile-modified cellulosic nanomaterial (e.g., CNCs and/or CNFsfunctionalized to contain a dienophile). The modulus of the compositematerial is reversibly controllable by adjusting a degree ofcrosslinking between the diene-modified cellulosic nanomaterial and thedienophile-modified cellulosic nanomaterial. This degree of crosslinkingis thermally reversible. On one hand, the degree of crosslinking may beincreased via a Diels-Alder (DA) cycloaddition reaction at a firsttemperature, thereby increasing the modulus of the composite material.On the other hand, the degree of crosslinking may be decreased via aretro-DA reaction at a second temperature higher than the firsttemperature, thereby decreasing the modulus of the composite material.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Embodiments of the present invention will hereinafter be described inconjunction with the appended drawings, where like designations denotelike elements.

FIG. 1 is a block diagram illustrating an exemplary printed circuitboard (PCB) having layers of dielectric material that incorporate asmart composite having a reversibly controllable modulus and containingmodified cellulosic nanomaterials in accordance with some embodiments ofthe present invention.

FIG. 2 is a block diagram illustrating an exemplary connector having aplastic housing and an exemplary plastic enclosure panel each of whichincorporates a smart composite having a reversibly controllable modulusand containing modified cellulosic nanomaterials in accordance with someembodiments of the present invention.

DETAILED DESCRIPTION

For purposes of this document, including the claims, the term “modulus”refers to the tensile modulus of a material. Tensile modulus is ameasure of the stiffness of a material. Tensile modulus is also known asYoung's modulus and elastic modulus.

There currently is no means to reversibly control the modulus of acomposite material after the material has been processed. Numerousapplications exist where it would be beneficial to have a low moduluscomposite material that can be easily deformed or compressed; yet inservice, it would be beneficial to increase the modulus of that samecomposite material (e.g., to prevent creep or stress relaxation). Forexample, a lower modulus state of a composite material may be beneficialwhen a product is being manufactured and/or reworked and/or maintained,and a higher modulus state of the composite material may be beneficialwhen the product is in service. Exemplary applications include, but arenot limited to, the composite materials used to produce printed circuitboards (PCBs), connectors, and enclosure panels.

Cellulosic nanomaterials (e.g., cellulose nanocrystals (CNCs) and/orcellulose nanofibrils (CNFs)) can be used as a filler to control therheology of numerous formulations as well as the physical properties ofcomposite materials incorporating them. Cellulosic nanomaterials arebio-derived (typically from trees, but may also be produced from otherplants). Cellulosic nanomaterials are also referred to as nanocellulose.Generally, cellulosic nanomaterials have a diameter between 5 nm to 500nm and a length between tens of nm to hundreds of μm. Many cellulosicnanomaterials are commercially available. Cellulosic nanomaterials mayalso be produced using techniques well known to those skilled in theart.

Cellulosic materials include, but are not limited to, CNCs and CNFs.CNCs and CNFs are two different colloidal forms of cellulose. CNCs andCNFs are prepared from pulp fibers, typically from trees. CNCs aretypically prepared through acid hydrolysis of pulp fibers. CNCs arecommercially available from suppliers such as CelluForce (Montreal,Canada). For example, CelluForce offers CNCs with specified averagedimensions of 5 nm diameter and 100 nm length. CNFs are typicallyprepared through mechanical disintegration of pulp fibers. CNFs arecommercially available from suppliers such as Rayonier Inc.(Jacksonville, Fla., USA) and Daicel FineChem Ltd. (Tokyo, Japan). Therespective shape of CNCs and CNFs may be compared to “rice” and“spaghetti”.

CNCs, CNFs and other cellulosic nanomaterials used as a filler in acomposite material undergo H-bonding to form a network of dispersedparticles. However, hydrogen bonds are weak and can be easily ruptured.In accordance with some embodiments of the present invention, individualCNCs, CNFs or other cellulosic nanomaterials are covalently linked ondemand. By doing so, the modulus of the composite material can bealtered (i.e., formation of the covalent bond results in a dramaticincrease in modulus). By functionalizing CNCs, CNFs or other cellulosicnanomaterials to contain either a diene or a dienophile, in accordancewith some embodiments of the present invention, Diels-Alder chemistrymay be utilized to crosslink/uncrosslink the CNCs, CNFs or othercellulosic nanomaterials.

An exemplary synthetic procedure to modify CNCs with either a diene(Reaction Scheme 1) or a dienophile (Reaction Scheme 2) is illustratedbelow. The reaction between the diene and the dienophile occurs at 110 Cto crosslink the CNCs via a Diels-Alder (DA) cycloaddition reaction(Reaction Scheme 3, Upper Arrow) as illustrated below. The crosslinkbetween the CNCs may be reversed via a retro-DA reaction at 130 C(Reaction Scheme 3, Lower Arrow) as illustrated below.

In the exemplary synthetic procedure illustrated in Reaction Scheme 1, adiene-modified CNC is synthesized by reacting hydroxyl groups on thesurface of a CNC with a suitable diene, such as 2-(hydroxymethyl)furan,in the presence of thionyl chloride. This reaction occurs at roomtemperature. Stoichiometric quantities of the reactants may be used.That is, a stoichiometric quantity of 2-(hydroxyl methyl)furan may reactwith all of the hydroxyl groups on the surface of the CNC. On the otherhand, it may be desirable to adjust the degree of diene-modification ofthe CNC by, for example, reacting less than the stoichiometric quantityof 2-(hydroxyl methyl)furan relative to the number of hydroxyl groups onthe surface of the CNC. Adjustment of the degree of diene-modificationof the CNC may be used to control the resulting modulus of the compositematerial. That is, the degree of diene-modification of the CNC may beused to control an upper limit relative to the number of diene groupsavailable for the subsequent Diels-Alder cycloaddition reaction(Reaction Scheme 3), described below.

The exemplary synthetic procedure to synthesize the diene-modified CNCillustrated in Reaction Scheme 1 is set forth for purposes ofillustration and not limitation. Any suitable diene-modified cellulosicnanomaterial (e.g., CNC and/or CNF) may be synthesized using techniqueswell known in the art. For example, the reaction utilized in ReactionScheme 1 to couple the diene moiety to the surface of the CNC may bereplaced with any suitable coupling chemistry to couple any suitablediene moiety to the surface on any suitable cellulosic nanomaterial. Forexample, alkoxysilanes or chlorosilanes can be condensed on the surfaceof a cellulosic nanoparticle to yield a cellulosic nanoparticlecontaining numerous pendant diene groups. Cellulosic nanomaterialsurfaces offer numerous possibilities for surface modification fromwhich to choose a suitable coupling chemistry. For example, the surfaceof a cellulosic nanoparticle may be functionalized to contain afunctional group such as isocyanate, vinyl, amine, or epoxy.

Suitable diene moieties include, but are not limited to, furans,pyrroles, or thiophenes.

In the exemplary synthetic procedure illustrated in Reaction Scheme 2, adienophile-modified CNC is synthesized using two steps. In the firststep of Reaction Scheme 2, a suitable dienophile, such as7-oxabicyclo[2.2.1]hept-5-ene-2,3-dicarboxy-N-(2-aminopropyl)imide, issynthesized. For example,7-oxabicyclo[2.2.1]hept-5-ene-2,3-dicarboxy-N-(2-aminopropyl)imide maybe synthesized by reacting furan with N-(2-aminopropyl)maleimide in thepresence of acetone at 55 C. Generally, stoichiometric quantities of thereactants may be used. In the second step of Reaction Scheme 2, adienophile-modified CNC is synthesized by reacting hydroxyl groups onthe surface of a CNC with a suitable dienophile, such as7-oxabicyclo[2.2.1]hept-5-ene-2,3-dicarboxy-N-(2-aminopropyl)imide, inthe presence of thionyl chloride. This reaction occurs at roomtemperature. Stoichiometric quantities of the reactants may be used.That is, a stoichiometric quantity of7-oxabicyclo[2.2.1]hept-5-ene-2,3-dicarboxy-N-(2-aminopropyl)imide mayreact with all of the hydroxyl groups on the surface of the CNC. On theother hand, it may be desirable to adjust the degree ofdienophile-modification of the CNC by, for example, reacting less thanthe stoichiometric quantity of7-oxabicyclo[2.2.1]hept-5-ene-2,3-dicarboxy-N-(2-aminopropyl)imiderelative to the number of hydroxyl groups on the surface of the CNC.Adjustment of the degree of dienophile-modification of the CNC may beused to control the resulting modulus of the composite material. Thatis, the degree of dienophile-modification of the CNC may be used tocontrol an upper limit relative to the number of dienophile groupsavailable for the subsequent Diels-Alder cycloaddition reaction(Reaction Scheme 3), described below.

The exemplary synthetic procedure illustrated in Reaction Scheme 2 isset forth for purposes of illustration and not limitation. Any suitabledienophile-modified cellulosic nanomaterial (e.g., CNC and/or CNF) maybe synthesized using techniques well known in the art. For example, thereaction utilized in Reaction Scheme 2 to couple the dienophile moietyto the surface of the CNC may be replaced with any suitable couplingchemistry to couple any suitable dienophile moiety to the surface on anysuitable cellulosic nanomaterial. For example, alkoxysilanes orchlorosilanes can be condensed on the surface of a cellulosicnanoparticle to yield a cellulosic nanoparticle containing numerouspendant dienophile groups. Cellulosic nanomaterial surfaces offernumerous possibilities for surface modification from which to choose asuitable coupling chemistry. For example, the surface of a cellulosicnanoparticle may be functionalized to contain a functional group such asisocyanate, vinyl, amine, or epoxy.

Suitable dienophile moieties include compounds having a di-substitutedalkene bearing electron withdrawing groups on both sides of the doublebond. Suitable electron withdrawing groups include, for example, ester,amide or keto groups. Dienophiles also include compounds which contain abut-2-ene-1,4-dione moiety that are contained in a 5- or 6-memberedring. For example, the dienophile may be a maleimide (i.e., a 5-memberedring) moiety. Examples of other suitable dienophiles include,bis(triazolinediones), bis(phthalazinediones), quinones,bis(tricyanoethylenes) bis(azodicarboxylates), diacrylates, maleate orfumarate polyesters, acetylenedicarboxylate polyesters.

A crosslinked network can be generated by simply heating the modifiedCNCs at 110 C, in accordance with some embodiments of the presentinvention, to form the crosslinked structure illustrated in ReactionScheme 3 (Upper Arrow) via the Diels-Alder cycloaddition reaction. Thiscrosslinked structure is referred to as a Diels-Alder adduct. Although asingle crosslink or Diels-Alder adduct is illustrated in Reaction Scheme3, one skilled in the art will appreciate that numerous sites along theCNC are modified so that each CNC may be crosslinked to any number ofother CNCs. Heating the composite material at 110 C induces theformation of Diels-Alder adducts and thereby generates a crosslinkednetwork. This enables the ability to control the resulting modulus of acomposite material into which the modified CNCs are blended simply byadjusting the degree of modification of the different CNCs, as describedabove, as well as the residence time at 110 C. The crosslinked networkgenerated by heating the modified CNCs at 110 C remains after thecomposite material cools below 110 C. When it is desired to decrease themodulus, the composite material is heated to 130 C, in accordance withsome embodiments of the present invention, to facilitate the retro-DAreaction illustrated in Reaction Scheme 3 (Lower Arrow). When it isdesired to once more increase the modulus, cooling the compositematerial to a temperature under 130 C again induces the formation ofDiels-Alder adducts and thereby regenerates the crosslinked network.

The Diels-Alder (DA) cycloaddition reaction and the retro-DA reactionillustrated in Reaction Scheme 3 are set forth for purposes ofillustration and not limitation. In accordance with some embodiments ofthe present invention, the DA reaction and the retro-DA reaction mayoccur under any suitable reaction conditions between any suitablediene-modified cellulosic nanomaterial (e.g., CNC and/or CNF) and anysuitable dienophile-modified cellulosic nanomaterial (e.g., CNC and/orCNF).

A composite material in accordance with some embodiments of the presentinvention is prepared by blending a bio-derived filler into a polymer,wherein the filler includes different modified cellulosic nanomaterials(i.e., diene-modified cellulosic nanomaterial and dienophile-modifiedcellulosic nanomaterial). The modulus of the composite material isreversibly controllable by adjusting a degree of crosslinking betweendifferent modified cellulosic nanomaterials. This degree of crosslinkingis thermally reversible. On one hand, the degree of crosslinking may beincreased via a Diels-Alder (DA) cycloaddition reaction at a firsttemperature, thereby increasing the modulus of the composite material.On the other hand, the degree of crosslinking may be decreased via aretro-DA reaction at a second temperature higher than the firsttemperature, thereby decreasing the modulus of the composite material.

In accordance with some embodiments of the present invention thediene-modified cellulosic nanomaterial is combined with thedienophile-modified cellulosic material to form a homogeneous mixture.For example, the different cellulosic nanomaterials may be mixed usingsolvent mixing. One skilled in the art will appreciate that thedifferent cellulosic nanomaterials may be mixed using any suitabletechnique known to those skilled in the art. The mixture is then blendedinto a polymer. For example, the mixture may be blended with the polymerusing a twin screw extruder. One skilled in the art will appreciate thatthe mixture may be blended into the polymer using any suitable techniqueknown to those skilled in the art. Advantageously, the mixture is notset (i.e., not crosslinked) at room temperature because the crosslinkingDiels-Alder cycloaddition reaction typically has an onset temperaturewell above room temperature, e.g., about 90 C and higher. Similar toother thermosetting materials, this characteristic permits the mixtureto be blended into the polymer and thereby form the composite material.Heating the composite material to a temperature above the onsettemperature induces the formation of Diels-Alder adducts and therebyyields a crosslinked network. The crosslinked network is retained afterthe composite material cools below the onset temperature. TheDiels-Alder adducts undergo reversion to the diene-modified cellulosicnanomaterial and the dienophile-modified cellulosic nanomaterial in asubstantially uncrosslinked state via a retro-DA reaction at or above areversion temperature. The reversion temperature is greater than theonset temperature. Cooling the composite material back to a temperatureunder the reversion temperature again induces the formation ofDiels-Alder adducts and the crosslinked state returns.

In lieu of blending the mixture into the polymer at a temperature belowthe onset temperature, the mixture may be blended into the polymer at atemperature equal to or above the reversion temperature. Cooling thecomposite material to under the reversion temperature then induces theformation of Diels-Alder adducts and thereby yields a crosslinkednetwork.

Also, in lieu of blending the mixture into the polymer, thediene-modified cellulosic nanomaterial and the dienophile-modifiednanomaterial may be individually blended into the polymer. For example,the diene-modified cellulosic nanomaterial and the dienophile-modifiednanomaterial may be individually blended into the polymer using separatefeed hoppers attached to a twin screw extruder. One skilled in the artwill appreciate that the different cellulosic nanomaterials may beindividually blended into the polymer using any suitable technique knownto those skilled in the art.

The amount of the bio-derived filler (i.e., the diene-modifiedcellulosic nanomaterial and the dienophile-modified nanomaterial) usedin the composite material may be empirically determined based, at leastin part, on the desired modulus when the crosslinked network ofDiels-Alder adducts is formed. Formation of the crosslinked network ofDiels-Alder adducts can increase the modulus of the composite materialseveral orders of magnitude (as compared to the modulus of the samecomposite material before formation of the crosslinked network ofDiels-Alder adducts or after the Diels-Alder adducts undergo reversion).The increase in the modulus of the composite material depends on anumber of factors including, for example, the bio-derived fillerloading, the degree of modification (i.e., diene-modification anddienophile-modification) of the cellulosic nanomaterials, and the degreeof crosslinking between the diene-modified cellulosic nanomaterial andthe dienophile-modified cellulosic nanomaterial. One skilled in the artwill appreciate, however, that additional factors beyond the increase inthe modulus of the composite material may figure into the determinationof the amount of the bio-derived filler to use in the compositematerial. Such additional factors include, for example, the rheology ofcomposite material, as well as other physical properties of thecomposite material. Typically, the bio-derived filler loading (i.e.,combined weight of the diene-modified cellulosic nanomaterial and thedienophile-modified nanomaterial as a percentage of the total weight ofthe composite material) will be within the range of 1% to 50%.

Typically, the ratio of the weight of the diene-modified cellulosicnanomaterial with respect to the weight of the dienophile-modifiedcellulosic nanomaterial will be approximately 50:50. This ratio willvary, however, depending on a number of factors including, for example,the degree of diene-modification of the diene-modified cellulosicnanomaterial relative to the degree of dienophile-modification of thedienophile-modified cellulosic nanomaterial, as well as the molecularweight of the diene-modified cellulosic nanomaterial relative to themolecular weight of the dienophile-modified cellulosic nanomaterial.

The use of synthetic polymers from petroleum sources is widespread.Petroleum-derived synthetic polymers, such as polycarbonate (PC) andacrylonitrile butadiene styrene (ABS), can be found in nearly every itemwe use in our daily lives. There is a growing shift to prepare polymericmaterials from renewable feedstocks because petroleum is a finiteresource. The use of these renewable polymers is envisaged inapplications from disposable products to durable goods. Some bio-derivedpolymers are already being produced on a commercial scale (e.g.,polylactic acid (PLA)). PLA is a good candidate to replacepolycarbonates (PC) and PC blends (e.g., PC/ABS). Other bio-derivedpolymers, such as polyhydroxyalkanoate (PHA) and polybutylene succinate(PBS), are also good candidates.

A bio-derived filler that includes diene-modified cellulosic materialand dienophile-modified cellulosic material, in accordance with someembodiments of the present invention, may be blended with one or morepetroleum-derived polymers (e.g., acrylonitrile butadiene styrene (ABS))and/or one or more bio-derived polymers (e.g., polylactic acid (PLA),polyhydroxyalkanoate (PHA), polybutylene succinate (PBS),polyhydroxybutyrate (PHB), and the like) to reversibly control themodulus of the resulting composite material. For example, in accordancewith some embodiments of the present invention, a bio-derived fillerthat includes diene-modified cellulosic nanomaterial anddienophile-modified cellulosic nanomaterial may be blended with aconventional sheet molding compound (SMC) (e.g., fiberglass reinforcedepoxy) to reversibly control the modulus of the resulting compositematerial.

A bio-derived filler that includes diene-modified cellulosic materialand dienophile-modified cellulosic material, in accordance with someembodiments of the present invention, may also serve to increase therenewable content in the resulting composite material (as compared tothe use of conventional fillers).

An exemplary printed circuit board (PCB) implementation of the presentinvention is described below with reference to FIG. 1, while anexemplary connector implementation and an exemplary plastic enclosurepanel implementation of the present invention are described below withreference to FIG. 2. However, those skilled in the art will appreciatethat the present invention applies equally to any manufactured articlethat employs thermosetting polymers (also known as “thermosets”) orthermoplastics.

FIG. 1 is a block diagram illustrating an exemplary printed circuitboard (PCB) 100 having layers of dielectric material that incorporate asmart composite having a reversibly controllable modulus and containingmodified cellulosic nanomaterials in accordance with some embodiments ofthe present invention. Each layer of dielectric material may, forexample, comprise a composite material that includes a polymer, such asepoxy resin reinforced with fiberglass, and a bio-derived filler blendedinto the polymer, wherein the filler includes diene-modified cellulosicmaterial and dienophile-modified cellulosic material to reversiblycontrol the modulus of the composite material. In the embodimentillustrated in FIG. 1, the PCB 100 includes one or more module sites 105and one or more connector sites 110. The configuration of the PCB 100shown in FIG. 1 is for purposes of illustration and not limitation.

In accordance with some embodiments of the present invention, each layerof dielectric material of the PCB 100 may, for example, comprise acomposite material that includes a sheet molded compound (SMC) offiberglass reinforced epoxy into which is blended a bio-derived fillerthat includes diene-modified cellulosic material and dienophile-modifiedcellulosic material to reversibly control the modulus of the compositematerial.

FIG. 2 is a block diagram illustrating an exemplary connector 200 havinga plastic housing 205 and an exemplary plastic enclosure panel 210 thatincorporate a smart composite having a reversibly controllable modulusand containing modified cellulosic nanomaterial in accordance with someembodiments of the present invention. In the embodiment illustrated inFIG. 2, the connector 200 is configured to make electrical contact withthe connector site 110 (shown in FIG. 1) of the PCB 100. Also in theembodiment illustrated in FIG. 2, the connector 200 includes a cable215. The configuration of the connector 200 and the configuration of theplastic enclosure panel 210 shown in FIG. 2 are for purposes ofillustration and not limitation.

In accordance with some embodiments of the present invention, theplastic housing 205 of the connector 200 may, for example, comprise acomposite material that includes a polymer, such as liquid crystalpolymer (LCP), and a bio-derived filler blended into the polymer,wherein the filler includes diene-modified cellulosic material anddienophile-modified cellulosic material to reversibly control themodulus of the composite material.

In accordance with some embodiments of the present invention, theplastic enclosure panel 210 may, for example, comprise a compositematerial that includes a polymer and a bio-derived filler blended intothe polymer, wherein the filler includes diene-modified cellulosicmaterial and dienophile-modified cellulosic material to reversiblycontrol the modulus of the composite material. The polymer may be anysuitable petroleum-derived polymer and/or any suitable bio-derivedpolymer. Suitable petroleum-derived polymers include, but are notlimited to, polycarbonates (PC), acrylonitrile butadiene styrene (ABS),and blends thereof. Suitable bio-derived polymers include, but are notlimited to, polylactic acid (PLA), polyhydroxyalkanoate (PHA),polybutylene succinate (PBS), and blends thereof. Plastic enclosurepanels are often referred to as “thermoplastic covers”.

One skilled in the art will appreciate that many variations are possiblewithin the scope of the present invention. Several embodiments of thepresent invention are described above in the context of exemplaryapplications (e.g., printed circuit boards (PCBs), connectors, andenclosure panels). However, the present invention is also applicable toother applications. For example, some embodiments of the presentinvention are applicable to automotive body structure and chassiscomponents. Thus, while the present invention has been particularlyshown and described with reference to preferred embodiments thereof, itwill be understood by those skilled in the art that these and otherchanges in form and details may be made therein without departing fromthe spirit and scope of the present invention.

What is claimed is:
 1. A method of reversibly controlling a modulus of acomposite material, wherein the composite material comprises a polymerand a bio-derived filler blended into the polymer, wherein the fillercomprises a diene-modified cellulosic nanomaterial and adienophile-modified cellulosic nanomaterial, and wherein at least someof the diene-modified cellulosic nanomaterial and at least some of thedienophile-modified cellulosic nanomaterial are crosslinked to eachother in a crosslinked network, comprising: adjusting a degree ofcrosslinking between the diene-modified cellulosic nanomaterial and thedienophile-modified cellulosic nanomaterial by at least one of:increasing the degree of crosslinking between the diene-modifiedcellulosic nanomaterial and the dienophile-modified cellulosicnanomaterial via a Diels-Alder (DA) cycloaddition reaction at a firsttemperature; and decreasing the degree of crosslinking between thediene-modified cellulosic nanomaterial and the dienophile-modifiedcellulosic nanomaterial via a retro-DA reaction at a second temperaturehigher than the first temperature.
 2. The method as recited in claim 1,wherein the diene-modified cellulosic nanomaterial comprises at leastone of cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs)functionalized to contain a diene.
 3. The method as recited in claim 2,wherein the dienophile-modified cellulosic nanomaterial comprises atleast one of CNCs and CNFs functionalized to contain a dienophile. 4.The method as recited in claim 1, wherein the diene-modified cellulosicnanomaterial comprises cellulose nanocrystals (CNCs) functionalized tocontain a diene, and wherein the dienophile-modified cellulosicnanomaterial comprises CNCs functionalized to contain a dienophile. 5.The method as recited in claim 1, wherein the polymer selected from agroup consisting of polylactic acid (PLA), polyhydroxyalkanoates (PHA),polybutylene succinate (PBS), polycarbonate (PC), acrylonitrilebutadiene styrene (ABS), and combinations thereof.
 6. A method ofreversibly controlling a modulus of a composite material, wherein thecomposite material comprises a polymer and a bio-derived filler blendedinto the polymer, wherein the filler comprises diene-modified cellulosicnanocrystals (CNCs) and dienophile-modified CNCs, wherein thediene-modified CNCs comprise CNCs functionalized to contain a diene,wherein the dienophile-modified CNCs comprise CNCs functionalized tocontain a dienophile, and wherein at least some of the diene-modifiedCNCs and at least some of the dienophile-modified CNCs are crosslinkedto each other in a crosslinked network, comprising: adjusting a degreeof crosslinking between the diene-modified CNCs and thedienophile-modified CNCs by at least one of: increasing the degree ofcrosslinking between the diene-modified CNCs and the dienophile-modifiedCNCs via a Diels-Alder (DA) cycloaddition reaction at a firsttemperature; and decreasing the degree of crosslinking between thediene-modified CNCs and the dienophile-modified CNCs via a retro-DAreaction at a second temperature higher than the first temperature. 7.The method as recited in claim 6, wherein the diene-modified CNCs arerepresented by the following formula:


8. The method as recited in claim 7, wherein the dienophile-modifiedCNCs are represented by the following formula:


9. The method as recited in claim 8, wherein increasing the degree ofcrosslinking between the diene-modified CNCs and the dienophile-modifiedCNCs includes heating the composite material to 110° C. to induceformation of Diels-Alder adducts, and then cooling the compositematerial to below 110° C.
 10. The method as recited in claim 9, whereindecreasing the degree of crosslinking between the diene-modified CNCsand the dienophile-modified CNCs includes heating the composite materialto 130° C. to facilitate the retro-DA reaction.
 11. A method ofreversibly controlling a modulus of a composite material, wherein thecomposite material comprises a polymer and a bio-derived filler blendedinto the polymer, wherein the polymer selected from a group consistingof polylactic acid (PLA), polyhydroxyalkanoates (PHA), polybutylenesuccinate (PBS), polycarbonate (PC), acrylonitrile butadiene styrene(ABS), and combinations thereof, wherein the filler comprises adiene-modified cellulosic nanomaterial and a dienophile-modifiedcellulosic nanomaterial, wherein the diene-modified cellulosicnanomaterial comprises at least one of cellulose nanocrystals (CNCs) andcellulose nanofibrils (CNFs) functionalized to contain a diene, whereinthe dienophile-modified cellulosic nanomaterial comprises at least oneof CNCs and CNFs functionalized to contain a dienophile, and wherein atleast some of the diene-modified cellulosic nanomaterial and at leastsome of the dienophile-modified cellulosic nanomaterial are crosslinkedto each other in a crosslinked network, comprising: adjusting a degreeof crosslinking between the diene-modified cellulosic nanomaterial andthe dienophile-modified cellulosic nanomaterial by at least one of:increasing the degree of crosslinking between the diene-modifiedcellulosic nanomaterial and the dienophile-modified cellulosicnanomaterial via a Diels-Alder (DA) cycloaddition reaction at a firsttemperature; and decreasing the degree of crosslinking between thediene-modified cellulosic nanomaterial and the dienophile-modifiedcellulosic nanomaterial via a retro-DA reaction at a second temperaturehigher than the first temperature.
 12. The method as recited in claim 1,wherein the dienophile-modified cellulosic nanomaterial comprises atleast one of cellulose nanocrystals (CNCs) and cellulose nanofibrils(CNFs) functionalized to contain a dienophile.
 13. The method as recitedin claim 6, wherein the dienophile-modified CNCs are represented by thefollowing formula: